Optical communications systems, including optical components and networking/data transmission components are configured for maximizing speed and capacity for data communications. Wavelength division multiplexing (WDM) has been used in optically amplified communications systems for combining a number of data channels in parallel in the same fibre. Hence, bandwidth has been effectively increased without the requirement of any significant modification to the system. Those of skill in the art will understand that a channel can be associated with a specific frequency or band of frequencies, where the information transmitted through the channel is represented by a data stream of encoded signals.
To further increase capacity of optical communications systems, consideration has been given to the technique called polarization multiplexing. Known methods applied to long haul transmission bit-rates of 40 Gbps over optical links, such as fibres, include quadrature phase shift keying (QPSK) and differentially coded QPSK (DQPSK), both being types of modulation. As those of skill in the art will understand, QPSK is a form of modulation in which a carrier is sent in one of four phases per symbol, such as at 45, 135, 225, and 315 degrees for example, encoding two bits per symbol. In DQPSK the change in phase from one symbol to the next encodes two bits per symbol.
In a conventional 40 Gbps dual-polarization system, four 10 Gbps channels are independently forward error correction (FEC) encoded, and each channel can carry a data stream. FIG. 1 is a block diagram of a prior art dual-polarization transmitter 10, which illustrates how a final optical signal λ_T is generated from four individual channels, carrying data streams a, b, c, d. As shown in FIG. 1, each data stream is error encoded via respective forward error correction (FEC) blocks 12. FEC is a well-known technique for effective data transmission error control.
Error encoded data streams a and b are provided by FEC's 12 to symbol mapping logic 14, while error encoded data streams c and d are provided by FEC's 12 to symbol mapping logic 16. The output of symbol mapping logic 14 is fed to modulator 18, and the output of symbol mapping logic 14 is fed to modulator 20. The output of modulators 18 and 20 are then provided to a horizontal polarizer 22 and a vertical polarizer 24, respectively. Horizontal polarizer 22 generates linearly (horizontally) polarized QPSK symbols. Vertical polarizer 24 generates orthogonally (vertically) polarized QPSK symbols.
Although by way of illustration, FIG. 1 shows horizontal and vertical polarizers 22 and 24 respectively, the key requirement is that the polarizers 22 and 24 be orthogonal. For example, the polarizers can generate orthogonal right and left circularly polarized light. The orthogonally polarized signals are combined by adder 26 and transmitted in a single wavelength signal λ_T. The single wavelength signal λ_T is transmitted over an optical fibre cable to a conventional dual-polarization receiver, such as the dual-polarization receiver 30 shown in FIG. 2. It should be noted that symbol mapping logic 14/16, modulators 18/20, polarizers 22/24 and adder 26 form a signal processing block responsible for converting the outgoing data streams into an optical signal for transmission over an optical medium.
The single wavelength signal λ_T is transmitted over an optical fibre cable to a conventional dual-polarization receiver, such as the dual-polarization receiver 30 shown in FIG. 2. The receiver 32 receives the single wavelength signal λ_T to extract the orthogonal polarized QPSK symbols, and separate the four individual FEC encoded data streams from each other. Essentially, dual-polarization receiver 30 reverses the signal processing executed by dual-polarization transmitter 10.
The dual-polarization receiver 30 includes depolarizing circuit 32, optical-to-electrical converters 34/36, symbol de-mapping logic 38/40, and FEC decoder blocks 42. The depolarizing circuit 32 provides two symbol mapped output signals to optical-to-electrical converters 34 and 36. The electrical signals generated by optical-to-electrical converters 34 and 36 are provided to symbol de-mapping logic 38 and 40 respectively. Symbol de-mapping logic 38 provides a pair of FEC encoded data streams to its FEC decoders 42, to generate original data streams a and b, while symbol de-mapping logic 40 provides a pair of FEC encoded data streams to its FEC decoders 42, to generate original data streams c and d.
Polarization multiplexed systems are subject to polarization dependent effects (PDE's), such as Polarization Dependent Loss/gain (PDL), Polarization Mode Dispersion (PMD), and other types of well known effects. PDL in particular, is a form of signal degradation induced intrinsically by the physical characteristics of the fibre itself, and/or induced externally through transient changes in the polarization couplings along a fibre route.
Unfortunately, PDL does not affect the linearly and orthogonally polarizations equally, thus potentially resulting in significant performance variations between the two polarizations. For example, as shown in the plot of bit error rate (BER) vs signal-to-noise ration (SNR) of FIG. 3 for a dual polarization system, the horizontal (linear) polarization component 26 has lower BER vs SNR relationship than the vertical (orthogonal) polarization component 28. Hence the BER on each of the transmitted polarizations is different. In particular, the BER of signals transmitted over different polarizations can differ by orders of magnitude. System users typically set limitations on transmission power levels and BER. It is noted that BER can be improved by increasing the transmission power level, but this improvement is bounded by the limits on system transmission power levels. However, overall system performance is based on the worst case performance characteristics associated with each polarized component. As a result, the performance advantages of the orthogonal polarized component cannot be fully exploited.
It is, therefore, desirable to provide a method and system for improving performance of polarization multiplexed optical systems while minimizing the effects of PDL and minimizing modifications to the optical system infrastructure.